1. Field of the Invention
The invention relates to a MultiBand-Orthogonal Frequency Division Control (MB-OFDM) system, and more particularly to the automatic gain control (AGC) of a MB-OFDM system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) has been used for 40 years, and has been adopted by many current communication standards, such as IEEE 802.11a/g/n, ADSL, WiMax, DAB and DVB. The bandwidth of a MultiBand-OFDM (MB-OFDM) system, ranging from 3.1 GHz to 10.6 GHz, is divided into 14 sub-bands, each of which has a 528 MHz bandwidth. The sub-bands with smaller bandwidth can lower the design complexity of a baseband receiver to reduce system manufacture cost, and improve bandwidth manipulation flexibility of the whole system.
FIG. 1 shows an example of OFDM symbols transmitted in a MB-OFDM system. Because time-frequency interleaving (TFI) is assumed to be performed across only three sub-bands in FIG. 1, only three sub-bands of channel 122, 124 and 126 are shown, and each channel has a 528 MHz bandwidth. The first OFDM symbol 104 is first transmitted on channel 122, the second OFDM symbol 108 is then transmitted on channel 124, the third OFDM symbol 112 is then transmitted on channel 126, a fourth OFDM symbol is then transmitted on channel 122 again, and so on. Each symbol lasts 242.42 ns, which contains 128 samples at the sampling rate of 528 MHz. A zero-padding suffix is inserted at the end of each OFDM symbol, such as 106, 110, and 114. Each zero-padding suffix lasts 70.08 ns and contains 37 samples. The zero-padding suffix is inserted to ensure sufficient time for the transmitter and receiver to switch to the next channel. The switch of channels in a MB-OFDM system is called “frequency hopping”.
Automatic Gain Control (AGC) is a mechanism for controlling the signal gain of a baseband receiver. Because the signal strength is attenuated when the signal is transmitted between a transmitter and a receiver, the receiver has to amplify the signal to a tolerable power level according to an AGC mechanism before the signal is further processed. Thus, the power of a preamble of the received signal must be measured, so that the AGC can determine the gain magnitude to amplify the signal. The short preamble of an ordinary OFDM system includes 10 consecutive short symbols, with no problems arising when the short preamble is measured in the ordinary OFDM system.
In the MB-OFDM system, however, the preamble contains 18 symbols with zero padding suffixes inserted between the symbols. FIG. 2 shows the signal strength of the preamble received by a MB-OFDM receiver. A symbol 212 of the preamble is confined within the symbol boundaries 204 and 206, and the samples of the symbols between the symbol boundaries are measured, since there is no signal but only noise in the zero-padding suffixes, such as zero-padding suffix 210. If the zero-padding suffixes are measured by the AGC mechanism, the measured power indicates only the noise power, and the gain magnitude determined by the AGC mechanism is inaccurate. The performance of the receiver will degrade if the zero-padding suffixes of the preamble are measured by the AGC mechanism.